1. Field of the Invention
This invention relates generally to the field of semiconductor processing and more particularly to an interferometric photolithography method for producing contact hole patterns in photosensitive material.
2. Description of the Related Art
The fabrication of microelectronic integrated circuitry generally involves the patterning of device structures and layouts on a semiconductor substrate. The accepted practice for creating the requisite pattern is to first form a replica of the pattern on a mask (not necessarily in its final size) and then to transfer the mask pattern to a layer of photoresistive material (positive or negative photoresist being acceptable) formed on the semiconductor substrate. The transfer is accomplished by an optical photolithographic process, shining light of a certain wavelength through the mask and onto the photoresist, using whatever optical lenses are required to replicate the pattern in its proper size on the photoresist. Once the pattern has been transferred to the photoresist, the photoresist is processed to selectively remove portions of the pattern and expose the substrate below. The substrate, itself, can then be etched by, for example, an anisotropic plasma etch, or otherwise processed as required.
With the progressive shrinking of device sizes to as small as tenths of a micron, the dimensions of patterns to be transferred by optical photolithography are approaching the wavelengths of the optical radiation being used to effect the transfer. As this occurs, maintaining both a high pattern resolution and a depth of field sufficient to allow precise focusing on a substrate of imperfect planarity becomes a serious problem that requires the use of sophisticated mask designs, such as phase shifting masks. Another way of avoiding this problem, at least when the pattern to be transferred has a certain appropriate shape or periodicity, is by the use of interferometric photolithography rather than optical lithography. In interferometric lithography the pattern is directly formed by the standing wave interference pattern of two or more coherent optical beams rather than by the use of transmissive or reflective optical systems to form images of patterns in masks by the focusing of light rays. In short, the interference pattern becomes the transferred pattern. Two advantages of this approach are: 1) the dimensions of patterns that can be produced by wave interference is on the order of fractions of a wavelength and 2) the lack of lens focusing eliminates the problem of depth of field. A disadvantage of this approach is that only patterns produced by wave interference can be transferred to a substrate, thus limiting the selection of transferable patterns.
One type of pattern that can be quite appropriately formed by interferometric methods is a pattern of small holes such as those used for forming vias in VLSI circuitry. Brueck et al. (U.S. Pat. No. 5,759,744) discloses a method for forming a regular array of holes by crossing (at 90°) two patterns of linear interference fringes produced by intersecting laser beams. The intersections of the fringes produce a light intensity of sufficient strength to develop a layer of photoresist and form the regular array. In an attempt to improve the diversity and resolution of images produced by interferometric photolithography, Brueck et al (U.S. Pat. No. 6,042,998) disclose a method for extending the available spatial frequency content of an image by layering a plurality of interferometrically produced patterns in different photoresist layers. Again, Brueck et al., in (U.S. Pat. No. 6,233,044) disclose a method of combining optical and interferometric photolithography, wherein the interferometric part images the high frequency components of the pattern and the optical part images the low frequency components. Finally, Mermelstein (U.S. Pat. No. 6,140,660) discloses a method for interferometrically forming a non-periodic pattern by means of a synthetic aperture system, which is a plurality of beams controlled by a plurality of beam controllers, which together can create arbitrary overlaps and corresponding interference patterns.
Although the patents cited above offer an indication of the usefulness of interferometric photolithography, with the exception of U.S. Pat. No. 5,759,744 of Brueck et al., the techniques disclosed are highly complex. On the other hand, the aforementioned method of Brueck is applicable only to the patterning of a regular pattern of holes, so it is fairly limited in its scope. The purpose of the present invention is to provide a method of using interferometric photolithography that is both simple and applicable to a wider range of patterns.